Trihybrid cross and determining genotypic ratios

Well, the problem tells you what kind of birds you have, and you have to recognise first the genotype of the birds, then make the breedings and then to understand what fenotypes are given by the genotypes.
Here's the solution to your problem:
Write the genotypes of the birds: the chicken that is true-breeding for feathered legs and single comb has FFpp, and the chicken that is true-breeding for the absence of leg feathers and pea comb is ffPP. Their chicks will all have the genotype FfPp (this is the F1 generation, the first breeding). In F2, they will give:
FFPP, FfPP, FFPp, FfPp
FfPP, ffPP, FfPp, ffPp
FFPp, FfPp, FFpp, Ffpp
FfPp, ffPp, Ffpp, ffpp.
(16 chicks)--From these, you only keep the ones with single comb and feathered legs, that means you only keep the ones with the genotype FFpp or Ffpp. You have two Ffpp and one FFpp.
These birds will give, as shown, chicks:
FFpp, Ffpp, FFpp, Ffpp
FFpp, Ffpp, FFpp, Ffpp
FFpp, Ffpp, Ffpp, ffpp
Total: 5 FFpp, 6 Ffpp and 1ffpp.
That means:
-5/12 (41.66%) chickens with feathered legs and single comb (true breeding)
-6/12 (50%) chickens with feathered legs and single comb (true breeding only for the shape of the comb)
-1/12 (8.33%) chickens with featherless legs and single comb (true breeding).

And that's it!